MATTER, AND ITS GENERAL PROPERTIES. 



15 



What Is the 

 evidence of 

 the existence 

 of Pores in 

 all matter ? 



Jio. 1. If vre suppose the atoms of matter to consist of 



minute spheres or globes, it is obvious that it will be 

 impossible for them to come into perfect contact at 

 all points : so that there must be small spaces be- 

 tween them, where they do not touch each other. 

 Fig. 1 represents the manner in which we may im- 

 agine a collection of such atoms to be arranged to 

 form a crystal. 



15. The reasons for believing that the 

 atoms or particles of matter do not ac- 

 tually touch each other, are, that every 

 form of matter, so far as we are ac- 

 quainted with it, can by pressure be 

 made to occupy a smaller space than it origin- 

 ally filled. Therefore, as no two particles of 

 matter can occupy the same space at the same 

 time, the space, by which the size or volume of 

 a body may be diminished by pressure, must, before sucb 

 diminution took place, have been filled with openings, or 

 pores. Again, all bodies expand or contract under the 

 influence of heat and cold. Now, if the atoms were in ab- 

 solute contact with, each other, no such movements could 

 take place. 



The porosity of bodies is sometimes illustrated and explained 

 by reference to a sponge, which allows the cavities which per- 

 vade it to be filled with water, or some other fluid. Such an 

 illustration is not strictly correct. The cavities of a sponge are 

 not really its pores, any, more than the cells of a honey-comb 

 are the pores of wax. In common speech, however, the term pore is often 

 used to designate those openmgs which exist naturally in the substance of a 

 body, which are sufficiently large to admit of the passage of fluids like water, 

 and gases like air. 



Several very important properties of matter are dependent on porosity ; or, 

 In other words, they owe their existence to the fact, that the particles of mat- 

 ter do not actually touch each other. The principal of these are Density, 

 Compressibility, and Expansibility. These properties of matter belong to 

 all bodies, but not to all alike. 



16. By Density we mean the proportion 

 which exists between the quantity of matter 

 contained in a body and its magnitude, or size. 

 Thus, if of two substances, one contains twice as much 



What is gen- 

 erally meant 

 by the term 

 Pores? 



What is Dens- 

 ity? 



